edited bv Denisan University Granville. Ohio 43023
The Iodine Clock Reaction: A Surprising Variant W~lrrar ev
Marcos Albedo Autuorl, Alexandra Gulmarses Bmlo, and Alfred0 Luls M. L. Mateus Unlrersldade de S h Paul0 P.O. Box 20780 01498-Siio Paulo-SP, B r a d
CHECKED eY
Wayne L. SmHh Colby Collsga
Waterrllla, ME 04901 T h e iodine clock reaction is one of the most classical chemical demonstrations of all, and i t has appeared in this Journal several times (1,2),sometimes in variations such as the Old Nassau (3). The highlights of these reactions are:
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They are ideal for intraducing chemical kinetics concepts and inorganic reactions mechanisms. They show very good rate dependence on concentration and temperature. The iodine dock is inexpensive, safe, and easy to prepare and perform.
Mast important, i t is always surprising and fascinating! Everyone who has seen the "William Tell Overture" knows i t
acid instead of sulfuric in solution C. and after the mixture turns dark hlue the color begins to fade sl&vly.The malonic arid reacts with iodine in the same way as in the Rriggs-Rauscher oscillating reaction (5).Mechanistic details can he found in ref 6. ~
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Certainly your audience will be very surprised by this hehavior. Once more the interest of your students will have awaked, and you can introduce topics on organic chemistry mechanisms, besides other kineties aspects. The present variation can he extended to the "William Tell" version (4) hy adding malonic acid to the solutions. As the music is played, a "colorless wave" follows the hlue one.
Acknowledgment The authors wish to thank Henriqne Eisi Toma and Jose Atilio Vanin for helpful suggestions. LHarature CHed 1. Lambed, J. L.; FinqG.T.J. Chem. Edue. 1984.61.1037-1038. 2. R w l , R. II: S~4trer.R.W. J. Ckm. Edur. 1987, M,446. 3. M-, A. J. Ckm. E d u . 1378.J5. -245. 4. Briee.L. KJ. Chem.Educ. 1980.57, 152. 5. Brigg8.T. S.:Rauaeher, W. C . J . Ckm. Ed-. t973,50,486. 6. L e o ~ l dK. , R; Haim,A. Int. J. Chmm. Kin& 1977.3.83-95.
(4). As a matter of fact, surprise is one of the most important features of a good demonstration, since i t enhances the student's interest in the subject being presented. I n the present note, we introduce a new version of the iodine clock reaction that aims t o draw the attention to something that students cannot immediately explain. Once their curiosity has been aroused, the theory underlying can he explained a t any level you want.
Solutions Classical iodine clock (4) A: potassium iodate 2 giL sulfuric acid 0.03 M B: sodium hydrogen sulf~te0.40 g in starch (2 g n ) previously dissolved in boiling water Variant . C: potassium iadate 2 g/L malonic acid 10 g n In the usual clock (Landolt)use equal volumes of A and B (4);in the variant use equal volumes of B and C. ~~~
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Procedure Firat perform the classid iodine clock by telling the audience that you re going to do a very simple experiment: you will mix two colorless solutions, and the mixture will turn dark hlue. You tell the audience: "It's easy! It always works!" Actually it takes some time for the reaction to occur. Intraducing a little hit of showmanship, you may look very ashamed and confused. Then, when the color changes, tell your audience that, believe it or not, you knew previously it would take a little time for the reaction to occur, because chemical reactions are not all instantaneous nor simple one-step processes. You can explain the mechanisms and other aspects of the reaction to the audience. This is the way we have presented this demonstration for over a hundred times in our Chemistry Show at the Science Museum in SHo I'aulo, Brazil. Now the present variation is introduced. You perform the demonstration again. hut now using solutions B and C. We used malonic ~~~~~~~
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SusMmED BV
S. K. Mlshra, P. Parasher, and P. D. Sharma Unlrarch ol Ralastkan Jalpur-$02004, lndla CHEWEDBY
Experlmenlal
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Refractive Index-A Simple Demonstration Experiment
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Journal of Chemical Education
Thomas R. Tunle Brandels UnlversW Walham, MA 02254 Refractive index data fmd frequent applications in various fields of chemistry such as the identification of materials, determination of purity of samples, and quantitative analysis of solutions. Dipole momenta and molecular structure of various types of molecules are determined easily with the help of refractive index. Refractive index also helps in understanding the Tyndall phenomenon in colloids. It is, in fact, the refractive index that helps in distinguishing hetween two transparent objects. Though a n idea about the refractive index is given on the basis of the velocity of light through different media, the phenomenon can be easily understood with the help of the following demonstration experiment. Exparlmanl Materials benzene bromohenzene glass wool Procedure An approximately 2-in. thick layer of glass wool is set at the hottom of the glass test tuhe using a glass rod. The uniform setting of the glass wool hed is essential in order to achieve gwd results. On adding 40 mL of benzene to the test tuhe, the glass wool remains
clearly visihle. Add hramobenzene ur the henzene in the test tube, without disturbing the glasv wool, until the latter becomes invisible. The invisibility of the glass wool is achieved by adding -24 ml. of bromobenzene. Now stopper the test tube, and turn it upside down. The glass wool will remain where i t was and will be visible. The refractive index of benzene is 1.498, and that of bromobenzene is 1.557. When these two are taken toeether in a mixture in the volume proportion mentioned above, the refractive index of the mixture becomes 1.52, which matches that of theglass wool, making
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the latter invisible. Thus on the basis of the refractive index values, volume proportion can be calculated for both of the ronstituenta of the mixture. Thus, this bimple experiment helps in demonstration and understanding the significance of refractive index. Sometimesshaking thecontenteofthe test tube creates problems with the invisihiliry of the glass wool due to bubbles. This problem of thealass wool bccominesuffused with the bubbles can be avoided if the Former is pressed & place well with the glass rod during the pouring of the reagent mixture into the test tube.
Volume 66
Number 10
October 1989
853
